The present invention relates to plant GnTI sequences, in particular, plant nucleic acid sequences encoding the enzyme N-acetyl glucosaminyl transferase I (GnTI), as well as GntI antisense or sense constructs, deduced therefrom, and their translation products, antibodies directed against said translation products as well as the use of the sequence information for the production of transformed microorganisms and of transgenic plants, including those with reduced or lacking N-acetyl glucosaminyl transferase I activity. Such plants with reduced or lacking N-acetyl glucosaminyl transferase I activity are of great importance for the production of glyco-proteins of specific constitution with respect to their sugar residues.
In eukaryotes, glycoproteins are cotranslationally assembled in the endoplasmatic reticulum (ER) (i.e. during import into the ER lumen) by the attachment of initially membrane bound glycans (via dolichol pyrophosphate) to specific asparagine residues in the growing polypeptide chain (N-glycosylation). In higher organisms, sugar units located at the surface of the folded polypeptide chain are subjected to further trimming and modification reactions (ref. 1) in the Golgi cisternae. Initially, typical basic Glc3Man9GlcNAc2 units of the high-mannose type are formed by means of different glycosidases and glycosyl transferases in the ER, which during the passage through the different Golgi cisternae are subsequently converted to so-called complex glycans. The latter are characterized by less mannose units and the presence of additional sugar residues, such as fucose, galactose and/or xylose in plants and sialic acid (N-acetyl neuraminic acid, NeuNAc) in mammals (ref. 1,2,3). The extent of the modifications can differ between glycoproteins. Single polypetide chains may carry heterogeneous sugar chains. Furthermore, the glycosylation pattern may vary for a specific polypeptide (tissue specific differences), and does not always have to be uniform with respect to a specific glycosylation site, which is referred to as microheterogeneity (ref. 4,5). Up to now, the role of asparagine bound glycans is barely understood, which i.a. results from the fact, that said glycans may serve several functions (ref. 6). However, it can be assumed, that e.g. protection of a polypeptide chain from proteolytic degradation can also be achieved by glycans of a more simple oligomannosyl type (ref.7).
Glycoproteins are highly important in medicine and research. However, large scale isolation of glycoproteins is time-consuming and expensive. The direct use of glycoproteins isolated conventionally often raises problems, since upon administration as a therapeutic, single residues of the glycan components may cause undesired side effects. In this context, the glycan component predominantly contributes to the physico-chemical properties (such as folding, stability and solubility) of the glycoproteins. Furthermore, isolated glycoproteins, as already mentioned above, rarely carry uniform sugar residues, which is referred to as microheterogeneity.
For the production of glycoproteins for medicine and research, yeasts prove to be unsuitable, since they are only able to perform glycosylations of the so-called high-mannose type. While insects and higher plants exhibit complex glycoprotein modifications, these, however, differ from those of animals. Therefore, glycoproteins isolated from plants have a strong antigenic effect in mammals. In most cases, animal organisms with glycosylation defects are not viable, since terminal glycan residues (e.g. of membraneous glycoproteins) mostly possess biological signal function and are indispensable for cell-cell-recognition during the course of embryogenesis. Mammalian cell lines with defined glycosylation defects already exist, the cultivation of which, however, is labour-intensive and expensive.
For mammals, different glycosylation mutants have been described in detail at the cell culture level (ref. 7,8,9,10). Said mutants are either defective in biosynthesis of mature oligosaccharide chains attached to dolichol pyrophosphate or in glycan processing or show alterations in their terminal sugar residues, respectively. Some of these cell lines exhibit a conditional-lethal phenotype or are defective in intracellular protein transport. The consequences of said defects for the intact organism are difficult to estimate. It has been observed, that a modification in the pattern of complex glycans on the cell surfaces of mammals is accompanied by the formation of tumours and metastases, although a functional relationship could not yet unambiguously be demonstrated (ref. 9). Therefore, in mammals glycosylation mutants are very rare. These defects, summarized under HEMPAS (Hereditary Erythroblastic Multinuclearity with a Positive Acidified Serum lysis test) (ref. 10,11), are based either on a deficiency in mannosidase II and/or low levels of the enzyme N-acetyl glucosaminyl transferase II (GnTII), and have strongly limiting effects on the viability of the mutated organism. GntI knock-out mice, in which the gene for GnTI has been destroyed, already die in utero of multiple developmental defects (personal communication, H. Schachter, Toronto).
Until recently, no comparable mutants were known for plants. By the use of an antiserum, which specifically recognizes complex modified glycan chains of plant glycoproteins and which predominantly is directed against the highly antigenic xcex21xe2x86x922 linked xylose residues (ref. 12), the applicant was able to isolate several independant mutants from an EMS mutagenized F2 population of the genetic model plant Arabidopsis thaliana, which no longer showed complex glycoprotein modification (complex glycan, cgl mutants). After at least five back-crosses, each followed by intermittent selfings (to screen for the recessive defects), the glycoproteins were analyzed. These glycoproteins mainly carried glycans of the Man5GlcNAc2 type, indicating a defect in N-acetyl glucosaminyl transferase I (GnTI) (ref. 8). Indeed, the Arabidopsis cgl mutants lacked GnTI activity (ref. 13), which normally catalyzes the first reaction in the synthetic pathway to complex modified sugar chains (ref. 1). However, according to observations so far, the viability of the mutated plants is not affected. In recent publications, plants are suggested as a putative source for the production of pharmaceutically relevant glycoproteins or vaccines (ref. 14,15). There however, it was overlooked, that glycoproteins isolated from plants may cause severe immune reactions in mammals, which up to now obstructed the production of heterologous glycoproteins in cultivated plants.
The applicant could demonstrate by way of example for the Arabidopsis cgl mutant, that plants can manage without complex modified glycoproteins to a great extent (ref. 13). Initially, secretory proteins are normally glycosylated in the ER of the mutant. In the Golgi apparatus of the cgl mutant, however, the oligomannosyl chains linked to the polypetide backbone via asparagine residues (N-glycosylation) then remain at the stage of Man5GlcNAc2 residues, since N-acetyl glucosaminyl transferase I (GnTI) activity is missing (FIG. 1). By this bio-synthesis block, the plant specific complex glycoprotein modification and in particular the attachment of xcex11xe2x86x923 fucose and xcex21xe2x86x922 xylose residues is prevented, whereby the strong antigenic effect on the mammalian organism is absent. However, Arabidopsis as a herb only has little utilizable biomass. Therefore, for the large scale production of biotechnologically relevant glycoproteins these cgl plants are less suitable. As an alternative, cultivars, especially Solanaceae, such as potato, tobacco, tomato or pepper and furthermore alfalfa, canola, beets, soybean, lettuce, corn, rice and grain, with missing or highly reduced GnTI activity, would be ideal for the production of heterologous glycoproteins in plants. For this purpose, the methods of homology-dependent gene silencing would be applicable (ref. 16, 17).
As FIG. 3 demonstrates, the homology of the first determined plant GntI sequence from potato (Solanum tuberosum L., St) is extraordinary low in comparison to the corresponding known sequences of animal organisms (only 30-40% identity at the protein level, cf. FIG. 3A). Therefore, by the use of heterologous GntI gene sequences an efficient reduction of endogenous complex glycoprotein modification in plants by means of antisense or sense suppression, respectively, (ref. 21), probably cannot be achieved.
Thus, in medicine and research there is still the need for a cost-effective production in suitable organisms of recombinant glycoproteins with a minimum of uniform, i.e. defined sugar residues.
Nature of the present invention:
Since the applicant for the first time has been able to isolate and elucidate plant GntI cDNA sequences, it is now possible i.a. to obtain and, in particular, to generate any plant having reduced or missing GnTI activity, and to detect the corresponding mutants, respectively, by means of reverse genetic approaches following transposon (ref. 18) or T-DNA insertion (ref. 19), respectively, so as to produce glycoproteins with low antigenic potential in said mutants.
i) Enzymes
Generally, the present invention comprises different N-acetyl glucosaminyl transferase I enzymes (EC 2.4.1.101) from plants, e.g. potato (Solanum tuberosum L.), tobacco (Nicotiana tabacum L.) and Arabidopsis thaliana (L.). In particular, the present invention relates to enzymes, which exhibit or contain the amino acid sequences given in FIG. 2 and 3B as well as in the accompanying sequence protocol.
Further, the invention comprises enzymes, which are derived from amino acid sequences of the above mentioned enzymes by amino acid substitution, deletion, insertion, modification or by C-terminal and/or N-terminal truncation and/or extension, and whichxe2x80x94if showing enzymatic activityxe2x80x94exhibit a specificity comparable to that of the starting enzyme, i.e. N-acetyl glucosaminyl transferase I activity, and optionally a comparable activity.
In the present context, by the term xe2x80x9ccomparable activityxe2x80x9d an activity is understood, which is in the range of up to 100% above or below that of the starting enzyme. Accordingly, also comprised by the invention are derived enzymes or proteins with very low or completely lacking enzymatic activity, which is detectable by means of one or more of the tests mentioned as follows. The enzyme activity is determined by a standard assay, which is performed with microsomal fractions either under radioactive conditions, e.g. using UDP-[6-3H]GlcNAc as a substrate (ref. 13) or non-radioactive conditions (HPLC method; ref 20). Plant GnTI activity can be detected on the subcellular level in Golgi fractions (ref. 21). On account of low yields, however, it is almost impossible to enrich the enzyme from plants.
Alternatively, an enzyme derived according to the present invention, may optionally be defined as an enzyme, for which a DNA sequence encoding the enzyme can be determined or derived, which hybridizes to a DNA sequence encoding the starting enzyme or to a complementary sequence under stringent conditions, as defined as follows.
For example, an enzyme derived in such a manner represents an isoform, which comprises the amino acids 74 to 446 of the amino acid sequence illustrated in FIG. 2 and in SEQ ID No:1 and 2. This isoform i.a. lacks the membrane anchor formed by amino acids 10 to 29. As a result, this enzyme isoform may be located in the plant cytosol.
As examples for C- and/or N-terminally extended proteins, fusion proteins can be mentioned, comprising in addition to an amino acid sequence according to the invention a further protein, which e.g. exhibits a different enzymatic activity or which may be easily detected in another manner, such as by means of fluorescence or phosphorescence or on account of a reactivity with specific antibodies or by binding to suitable affinity matrices.
Furthermore, the invention comprises fragments of said enzymes, which optionally no longer exhibit any enzymatic activity. Generally, these fragments show an antigenic effect in a host immunized with said fragments, and may accordingly be employed as an antigen for the production of monoclonal or polyclonal antibodies by immunization of a host with those fragments.
Moreover, this invention also relates to N-acetyl glucosaminyl transferase I enzymes from other varieties and plant species, which are obtainable on account of hybridization of their genes or one or more regions of their genes:
to one or more of the DNA sequences and/or DNA fragments of the present invention, as discussed below and/or
to suitable hybridization probes according to the invention, which may be prepared on the basis of the amino acid sequences mentioned in the sequence protocol considering the degeneration of the genetic code.
Further comprised by the invention in accordance with the above are enzymes or proteins derived from these N-acetyl glucosaminyl transferase I enzymes, including fusion proteins thereof, as well as fragments of all of these enzymes or proteins.
ii) Antibodies
Another aspect of the present invention relates to the use of the amino acid sequences mentioned above and of fragments thereof having antigenic effects, respectively, for the production of monoclonal or polyclonal antibodies or antisera by immunizing hosts with said amino acid sequences or fragments, respectively, as well as of antibodies or antisera, respectively, per se, which specifically recognize and bind to the enzymes and/or antigens described above. The general procedure and the corresponding techniques for the generation of polyclonal and monoclonal antibodies are all well-known to the persons skilled in the art.
Exemplarily, by the use of a fragment of the GntI cDNA (nucleotides 275 to 1395) represented in FIG. 2 and SEQ ID NO: 1, the recombinant GnTI protein from Solanum tuberosum with 10 N-terminal histidine residues (His-tag) was overexpressed in E. coli, and, following affinity purification via a metal-chelate matrix, was employed as an antigen for the production of polyclonal antisera in rabbits (cf. Examples 5 and 6).
One possible use of the antibodies of the invention resides in the screening of plants for the presence of N-acetyl glucosaminyl transferase I.
Binding of the antibody according to the present invention to plant protein(s) indicates the presence of N-acetyl glucosaminyl transferase I enzyme detectable with said antibody. In general, this antibody may then be covalently bound to a carrier in a later step, and optionally be employed for the enrichment or purification of the enzyme by means of column chromatography.
On the other hand, a negative binding result using the antibody of the present invention, i.e. lack of binding to the plant proteins, may suggest, that N-acetyl glucosaminyl transferase I enzyme is absent (or highly modified by mutation), and thus, that N-acetyl glucosaminyl transferase I activity of a plant investigated is missing or highly reduced.
Techniques for the realization of the screening assays mentioned above or the enrichment or purification of enzymes by the use of antibody columns or other affinity matrices (cf. Examples 5 and 6) are well-known to those skilled in the art.
iii) DNA sequences
The present invention further comprises DNA sequences encoding the amino acid sequences of the invention, including amino acid sequences derived therefrom according to the above provisions. In particular, the invention relates to the respective gene, which is the basis of the amino acid sequences described in the FIGS. 2 and 3B and the sequence protocol, and especially, to the cDNA sequences described in FIG. 2 and the sequence protocol, as well as to DNA sequences derived from these genes and DNA sequences.
By the term xe2x80x9cderived DNA sequencesxe2x80x9d are meant sequences, which are obtained by substitution, deletion and/or insertion of one or more and/or smaller groups of nucleotides of the sequences mentioned above and/or by truncation or extension at the 5xe2x80x2 and/or 3xe2x80x2 terminus. Modifications within the DNA sequence may lead to derived DNA sequences, which encode amino acid sequences being identical to the amino acid sequence encoded by the starting DNA sequence, or to such sequences, in which, compared to the amino acid sequence, which is encoded by the starting DNA sequence, single or a few amino acids are altered, i.e. substituted, deleted and/or inserted, as well as to such sequences, whichxe2x80x94optionally in additionxe2x80x94are truncated and/or extended at the C-terminus and/or N-terminus.
Furthermore, the present invention also extends to the complementary sequences of the genes and DNA sequences according to the invention, as well as the RNA transcription products thereof.
Particularly comprised by the present invention are all sequences derived according to the above provisions, which over their entire length or only with one or more partial regions hybridize under stringent conditions to the starting sequences mentioned above or to the sequences complementary thereto or to parts thereof, as well as DNA sequences comprising such sequences.
By the term xe2x80x9chybridization under stringent conditionsxe2x80x9d in the sense of the present invention is understood a hybridization procedure according to one or more of the methods described below. Hybridizing: up to 20 h in PEG buffer according to Church and Gilbert (0.25 M Na2HPO4, 1M EDTA, 1% (w/v) BSA, 7% (w/v) SDS, pH 7.5 with phosphoric acid; ref. 22) at 42xc2x0 C. or in standard hybridization buffers with formamide at 42xc2x0 C. or without formamide at 68xc2x0 C. (ref. 23). Washing: 3 times at 65xc2x0 C. for 30 min in 3xc3x97SSC buffer (ref. 23), 0.1% SDS.
In the sense of the present application, the term xe2x80x9chybridizationxe2x80x9d always means hybridization under stringent conditions, as mentioned above, even if this is not explicitely indicated in the individual case.
Moreover, the invention relates to fragments of the DNA sequences mentioned above, including the DNA sequences derived in accordance with the above provisions, to fragments derived from such fragments by nucleic acid substitution, insertion and/or deletion as well as the corresponding fragments with sequences complementary thereto. Such fragments are i.a. suitable as sequencing or PCR primers, screening probes and/or for uses as discussed below. For the use as a screening or hybridizing probe, the DNA fragments according to the present invention are frequently employed as radio-labelled fragments. Fragments carrying sequences, which are derived from the starting sequences defined above by substitution, deletion and/or insertion of one or more nucleotides, and the sequences complementary thereto, respectively, are comprised by the invention to that extent, as said fragments hybridize under the above mentioned stringent conditions to the starting sequences, or to the sequences complementary thereto, respectively.
On the basis of the DNA sequences mentioned in the sequence protocol and in FIG. 2, DNA fragments according to the invention may for example be obtained starting from plant DNA by means of restriction endonucleases using appropriate restriction sites or by employment of PCR by means of primers appropriately synthesized, or may, as an alternative, also be chemically synthesized. Such techniques are well-known to those skilled in the art.
Moreover, the invention relates to any DNA sequences, which represent a gene or are a part of a gene encoding the enzyme N-acetyl glucosaminyl transferase I and, which in their entirety or in a partial region thereof hybridize under stringent conditions
to one or more of the DNA sequences of the invention and/or
to one or more of the DNA fragments of the invention and/or
to a DNA sequence, which is derived from the amino acid sequences mentioned in the sequence protocol considering the degeneration of the genetic code.
For this purpose, hybridization or screening probes are used as DNA fragments, which generally comprise at least 15 nucleotides, typically between 15 and 30 nucleotides, and, if necessary, substantially more nucleotides. As an example, the primers employed in Example 1 may be used. Alternatively, DNA sequences of appropriate length, derived from the DNA sequences mentioned in the sequence protocol, may be used. As a third possibility, appropriate hybridization probes according to the invention may be developed starting from the amino acid sequences mentioned in the sequence protocol considering the degeneration of the genetic code.
In this respect, a subject-matter of the present invention are also genes encoding N-acetyl glucosaminyl transferase I, which may be detected from other varieties or plant species on account of the hybridization thereof to above mentioned hybridization probes, as well as DNA sequences, DNA fragments and constructs, which are derived therefrom in accordance with the above provisions.
The isolation of the corresponding gene and sequencing thereof following detection by means of the hybridization probes of the invention are well within the skills of a specialist in this field, and are detailed by way of example with respect to N-acetyl glucosaminyl transferase I from Solanum tuberosum and to the corresponding enzymes from Nicotiana tabacum and Arabidopsis thaliana (partial sequence) in the examples.
Finally, another subject matter of the present invention are antisense sequences with respect to any of the above DNA sequences.
iv) Constructs
Also comprised by the invention are constructs, which may optionally comprise besides additional 5xe2x80x2 and/or 3xe2x80x2 sequences, e.g. linkers and/or regulatory DNA sequences or other modifications, the DNA sequences of the invention, including the DNA sequences derived as detailed above.
An example for this are hybridization or screening probes, which in addition to a DNA sequence of the invention also comprise a detection agent for the verification of hybridization products, which in this case typically is non-radioactive, e.g. fluorescent or phosphorescent molecules, biotin, biotin derivatives, digoxigenin and digoxigenin derivatives. In this respect, however, radioactive or non-radioactive detection agents may be considered, which may be attached to the DNA sequence according to the present invention e.g. by means of end labelling.
A subject-matter of the invention are also antisense and sense constructs with respect to the DNA sequences and fragments according to the present invention, i.e. with respect to
the DNA sequences mentioned in the sequence protocol and the corresponding genes;
the DNA sequences derived therefrom in accordance with the above provisions;
one or more regions of these DNA sequences;
DNA sequences, especially from other varieties or plant species, which represent a gene or are a part of a gene, encoding the enzyme N-acetyl glucosaminyl transferase I; and which hybridize under stringent conditions
to one or more of the above DNA sequences and/or
to one or more of the above DNA fragments and/or
to a DNA sequence, which is derived from the amino acid sequences mentioned in the sequence protocol considering the degeneration of the genetic code.
Furthermore, the present invention extends to any DNA-transfer systems such as vectors, plasmids, viral and phage genomes or cosmids, which contain the DNA sequences according to the present invention, e.g. the GntI gene, cDNA and DNA regions according to the invention, as mentioned in the sequence protocol, fragments thereof, in particular antisense or sense constructs and/or cDNA sequences derived therefrom according to the above provisions.
Various techniques for the production or synthesis of DNA, DNA fragments, constructs and transfer systems according to the invention, e.g. digestion by means of restriction endonucleases, PCR amplification using suitable primers, optionally followed by cloning and additional chemical or enzymatic modification starting from plant DNA are well-known to those skilled in the art.
One possibility of application of the DNA hybridization probes according to the invention is the detection of N-acetyl glucosaminyl transferase I genes in plants other than those, from which the DNA sequences mentioned in the sequence protocol were obtained, or the detection of potential (other) isoforms of the N-acetyl glucosaminyl transferase I gene in the starting plants Solanum tuberosum, Nicotiana tabacum and Arabidopsis thaliana. 
If it is possible to make use of a plant genomic library or cDNA library for the hybridization experiment, a positive hybridization result of such screening of each library may indicate a clone or a few clones, which contain the desired sequence completely or in part, i.e. the N-acetyl glucosaminyl transferase I gene, combined with only a limited amount of other DNA from the genome of the target plant, which appropriately facilitates cloning and sequencing of the target gene. As an alternative, a PCR amplification of the gene or parts thereof may also be carried out starting from plant DNA and suitable constructs, so-called PCR primers, to facilitate cloning and sequencing.
One use of sequencing primers of the invention, which are synthesized starting from suitable regions of the sequences according to the invention, e.g. enables genomic sequencing starting from the entire target plant genomic DNA cleaved by restriction endonucleases, by means of the Church-Gilbert technique, as well as sequencing at the cDNA level following RT-PCR amplification of the total RNA of the target plant (cf. Expl. 1).
An alternative possibility of application of the DNA hybridization probes according to the present invention derived from the DNA sequences mentioned in the sequence protocol, is the use thereof according to the invention for the detection of plants with reduced or lacking N-acetyl glucosaminyl transferase I activity. The hybridization experiment serves to detect the N-acetyl glucosaminyl transferase I (GntI) gene by which it may be concluded, e.g. owing to a negative hybridization result under stringent conditions, that the GntI gene, and thus, N-acetyl glucosaminyl transferase I activity in a plant investigated is lacking.
Such hybridization techniques for the detection of proteins or genes particularly in plant material by means of DNA probes are also known to the persons skilled in the art. In this context, it is referred to the above statements under item iii) for possible hybridization conditions. Generally, suitable DNA hybridization probes comprise at least 15 nucleotides of a sequence, which for example is derived from the cDNA sequences mentioned in FIG. 2 and the sequence protocol or from the corresponding GntI genes.
v) Transformed microorganisms
Furthermore, the invention relates to microorganisms, such as bacteria, bacteriophages, viruses, unicellular eukaryotic organisms, such as fungi, yeasts, protozoa, algae, and human, animal and plant cells, which have been transformed by one or more of the DNA sequences of the invention or one or more of the constructs of the invention, as illustrated above.
Transformed microorganisms according to the present invention are used e.g. as expression systems for the transforming foreign DNA to obtain the corresponding expression products. For this purpose, typical microorganisms are bacteria, e.g. such as E. coli. Furthermore, transformed microorganisms according to the invention, in particular agrobacteria, may be employed e.g. for the transformation of plants by transmission of the transforming foreign DNA.
Methods for the transformation of cells of microorganisms by (foreign) DNA are well-known to those skilled in the art.
For this purpose, e.g. constructs referred to as expression vectors are used, which contain the DNA sequence of the invention under control of a constitutive or inducible promoter, which, if necessary, is additionally tissue specific, so as to enable the expression of the introduced DNA in the target or host cell.
Therefore, a further aspect of the invention is a method for the production of the enzymes and proteins of the invention by using one or more of the transformed microorganisms of the present invention. The method comprises cultivating at least one microorganism transformed by the DNA of the invention, in particular by one of the cDNAs mentioned in the sequence protocol, under the control of an active promoter, as defined above, and isolating the enzyme of the invention from the microorganisms, and, if applicable, also from the culture medium. It is understood, that this method also relates to the production of enzymes and proteins, respectively, which are derived from the enzymes according to the present invention from Solanum tuberosum, Nicotiana tabacum and Arabidopsis thaliana, as defined under i) above.
Methods for the cultivation of transformed microorganisms are well-known to those skilled in the art. For example, the isolation of the expressed enzyme may be employed according to the method described in Example 5 by means of metal-chelate chromatography or, alternatively, by chromatography via columns, which contain the antibodies against the enzyme bound to the packing material.
vi) Transgenic plants
Furthermore, the invention comprises transgenic plants, which are transformed by means of a DNA sequence according to the invention or a corresponding construct, respectively. Accordingly, there may be obtained e.g. transgenic plants, in which a GnTI deficiency, for example on account of a missing or defectice GntI gene or due to defects in the regulatory regions of this gene, has been removed by complementation using a construct derived from the cDNA sequences mentioned in the sequence protocol, wherein the expression of said construct is under the control of an active constitutive or inducible promoter, which may be additionally tissue specific. In this case, the GnTI enzyme or protein expressed on account of the DNA of the invention contained in the construct and having GnTI activity complements the GnTI activity missing in the starting plant.
Also considered are transgenic plants, in which the GnTI activity already present in the starting plant is increased by additional expression of the GntI transgene introduced by means of a construct according to the present invention. Up to now, the extremely low expression of the GntI gene in vivo accompanied by extremely low enzyme activity, which correspondingly was very difficult to detect, has been a main problem in the investigation of the enzyme N-acetyl glucosaminyl transferase I in plants. The problem of a too low GnTI enzyme activity in plants may be overcome by the coexpression of a DNA according to the present invention.
In this case, it may be preferable for the transformation of plants to employ DNA according to the invention, additionally comprising a sequence region, which following expression enables a facilitated detection and/or enrichment and purification, respectively, of the protein product having GnTI activity. This is for example accomplished by the use of a specific DNA sequence for the expression of a recombinant GnTI enzyme, said sequence carrying a N-terminal or C-terminal sequence extension encoding an affinity marker. If it is additionally intended to provide an amino acid sequence portion between the GnTI enzyme and the affinity marker, which represents a recognition site for a specific protease, cleavage of the N-terminal or C-terminal sequence extension from the GnTI enzyme may be achieved by the subsequent use of this specific protease, and the GnTI enzyme thereby obtained in isolated form.
An example for this is the use of a DNA sequence according to the present invention, which codes for the recombinant GnTI enzyme with a C-terminal sequence extension, encoding the affinity marker AWRHPQFGG (strep-tag; ref. 39), and an intervening protease recognition site IEGR. The expression of the DNA according to the present invention provides GnTI enzymes with the C-terminal sequence extension mentioned, by means of which the expressed protein molecules specifically bind to a streptavidin derivatized matrix, and may thus be isolated. Then, by means of the protease factor Xa specifically recognizing the amino acid sequence IEGR, the GnTI portion of the protein molecules may be released. As an alternative, the complete protein may be removed from the streptavidin derivatized matrix by means of biotin or biotin derivatives.
A further example is represented by DNA sequences of the invention, encoding a protein which comprises multiple, e.g. 10, N-terminally added histidine residues (His-tag) in addition to a GnTI enzyme. Due to the N-terminal histidine residues, isolation or purification, respectively, of the proteins expressed may be easily conducted by metal-chelate affinity chromatography (e.g. Ni sepharose) (cf. also Example 5).
Moreover, the invention comprises portions of such transgenic plants, adequately transformed plant cells, transgenic seeds and transgenic reproduction material.
A further important aspect of the invention is the use of the sequence information discussed above for the production of plants having reduced or lacking N-acetyl glucosaminyl transferase I activity.
The possibilities of identifying plants with reduced or lacking N-acetyl glucosaminyl transferase I activity due to a gene defect or a missing gene by means of antibodies of the invention or screening or hybridization probes of the invention have already been described above.
Two additional possibilities reside in the use according to the invention of antisense or sense constructs, respectively, which are derived from the DNA sequence of a plant GntI gene, for the production of transgenic plants with reduced or lacking N-acetyl glucosaminyl transferase I activity by means of homology-dependent gene silencing (cf. ref. 16,17). The DNA sequence used as a starting sequence for the generation of the constructs, may be derived from the starting plant to be transformed itself but also from a different plant variety or species. In particular, antisense or sense constructs as discussed under items iii) and iv) above are of use. Generally, the constructs employed comprise at least 50 to 200 and more base pairs.
In particular, the constructs employed for this purpose comprise at least 50 to 200 and more base pairs, with a sequence, which is derived on the basis of
the cDNA sequences mentioned in the sequence protocol and/or the corresponding GntI genes and/or
the derived DNA sequences discussed above and/or DNA fragments according to the present invention and/or
the DNA sequences, in particular from other varieties and plant species, which encode N-acetyl glucosaminyl transferase I and which may be identified due to a hybridization under stringent conditions to hybridization or screening probes, as defined under items iii) and iv) above.
Generally, the constructs contain a strong constitutive or inducible promoter, which additionally may be tissue specific, by means of which the antisense or sense DNA sequence regions are controlled.
In the production of transgenic plants by integration of antisense construct(s) into the plant genome or by viral infection of starting plants or plant cells by means of virus containing antisense construct(s) for an extrachomosomal propagation and transcription of the antisense construct or the antisense constructs in infected plant tissue, it is intended to achieve a hybridization of GntI-gene transcripts to transcripts of the antisense DNA region at the RNA level, which prevents translation of the GntI mRNA. The result is a transgenic plant with strongly decreased contents of N-acetyl glucosaminyl transferase I, and thus, a strongly decreased corresponding enzyme activity.
For the transformation of plants according to the invention with antisense constructs, for example constructs may be employed, which hybridize to one of the complete cDNAs, mentioned in FIG. 2 and in the sequence protocol, or to corresponding regions thereof, generally comprising at least 50 to more than 200 base pairs. Moreover, particularly preferred is the use of fragments, the transcripts of which additionally cause a hybridization to a portion of the 5xe2x80x2 untranslated region of the GntI mRNA, at which or in the proximity of which usually the attachment of ribosomes would occur. Examples of such constructs are shown in FIG. 4.
In view of the occurence of an isoform in Solanum tuberosum, which probably is located in the cytoplasm due to lack of the membrane anchor (aa 10 to 29) of yet unkown function, it may be desirable to target only the N-acetyl glucosaminyl transferase I enzyme located in the Golgi cisternae, i.e. only that enzyme comprising the membrane anchor. One reason for this desire may be the effort or, in the individual case, also the requirement, to affect as little as possible the cytoplasmatic metabolism of the plant cell, for which the cytoplasmatic N-acetyl glucosaminyl transferase I possibly is of importance. For this purpose, antisense constructs may be used according to the present invention, which themselves or the transcripts of which, respectively, hybridize to a DNA or RNA region of the GntI gene or the GntI mRNA, comprising a part of the 5xe2x80x2 untranslated region and the coding region including the membrane anchor. Generally, the extension of the region of hybridization up to position 266 of the cDNA in FIG. 2 and SEQ ID NO: 1 is considered harmless for the purpose mentioned above.
In the production of transgenic plants by integration of sense constructs into the plant genome or by viral infection of starting plants or plant cells by means of virus containing sense construct(s) for extrachromosomal propagation and expression of the construct or constructs in infected plant tissue, there are assumed hybridization phenomena in tobacco according to the work of Faske et al. (ref. 17), of said constructs to the endogenous GntI gene at a posttranscriptional or DNA level, respectively, which finally affect or prevent the translation of the GntI gene. Also in this case, the result are transgenic plants having reduced or even lacking N-acetyl glucosaminyl transferase I activity.
Methods for the stable integration of such antisense and sense constructs into the genome of plants, or for the viral infection of plants or plant cells, respectively, for an extrachromosomal propagation and transcription/expression of such constructs in infected plant tissue are well known to those skilled in the art. This includes the direct DNA transfer (e.g. into protoplasts by means of electroporation or by the addition of a high molecular osmotic agent as well as biolistic methods, by which DNA coated particles are shot into the plant tissue), such as the use of natural host/vector systems (e.g. agrobacteria or plant viruses). For viral infection of starting plants or plant cells by viruses containing appropriate constructs for extrachromosomal propagation and transcription/expression of the constructs in infected plant tissue, a variety of specific viruses, such as tobacco mosaic virus (TMV) or potato virus X, is available.
Representative plants, which are suitable for such integration, comprise dicotyledonous as well as monocotyledonous cultivated plants, in particular Solanaceae such as potato, tobacco, tomato and pepper. Additionally, banana, alfalfa, canola, beets, soybean, lettuce, corn, rice and grain, would be suitable target plants for the use of homologous antisense constructs. For example, the sequence from Arabidopsis thaliana mentioned in the sequence protocol appears to be particularly suitable as a starting sequence for the transformation according to the invention of Brassicaceae, such as canola plants, by means of sense or antisense constructs. Further plants of interest are any plants, which express glycoproteins of interest for medicine and research.
Generally, it should be noted, that the transformation according to the invention of plants, which in the corresponding region of the GntI gene exhibit a homology of xe2x89xa770% at the nucleotide level to the employed antisense or sense constructs according to the present invention, typically results in transgenic plants of the invention, which show the desired reduction of N-acetyl glucosaminyl transferase I activity.
Further, another possibility is seen in the targeted destruction (knock-out) of the N-acetyl glucosaminyl transferase I gene via gene targeting by means of homologous recombination (ref. 24) in a target plant using a suitable DNA fragment derived from the cDNA sequence of the present invention, similar to the procedure established for yeast systems and mammals.
Further, the present invention comprises transgenic plants, which have been transformed by the antisense or sense constructs mentioned above or the viruses containing the same, respectively, as well as parts of such transgenic plants, correspondingly transformed plant cells, transgenic seeds and transgenic reproduction material.
Methods of the production of transgenic plants, e.g. by means of agrobacteria- or virus-mediated as well as direct DNA transfer are known to those skilled in the art. Concerning representative plants for such a transformation, the above mentioned applies.
The plants of the invention and the plants obtained according to the invention, respectively, with reduced or lacking N-acetyl glucosaminyl transferase I activity, may be used according to the invention for the production of glycoproteins with minimal and uniform, i.e. defined, sugar residues. As discussed above, such glycoproteins are of great importance for medicine and research. As a reasonable source of raw material and food as well as due to their unproblematical disposal via composting, plants per se represent ideal bioreactors. According to the present invention, it is now possible to express biotechnologically or pharmaceutically relevant glycoproteins (e.g. therapeutics of low antigenic potential for mammals) in cultivated plants, in which GnTI activity is highly reduced or completely absent.
Accordingly, the invention also comprises a method for the production of glycoproteins with minimal uniform and defined sugar residues, comprising cultivating a transgenic plant according to the invention, of parts of such plants or of plant cells transformed according to the invention, each expressing the desired glycoprotein, as well as isolating the desired glycoprotein from the cultivated material.
In this context, representative cultivated plants are Solanaceae, in particular potato, tobacco, tomato and pepper. Furthermore possible are banana, alfalfa, canola, beets, soybean, lettuce, corn, rice and grain.
The sequence of the enzymatically controlled and plant specific N-glycan modifications, which secretory glycoproteins are subjected to during passage through the Golgi apparatus of higher plants, is schematically shown in FIG. 1. The biosynthesis block due to lacking or insufficient N-acetyl glucosaminyl transferase I (GlcNAc transferase I) activity in a plant leads, instead of complex glycans, to the predominant formation of glycans of the Man5GlcNAc2 type, i.e. glycoproteins with uniform and well-defined sugar residues, which are of extremely high importance for medicine and research.
For this purpose, the genes encoding the desired glycoproteins may be expressed in their natural producing plants, which have been transformed according to the present invention e.g. by means of antisense or sense constructs to yield transgenic plants with reduced or missing N-acetyl glucosaminyl transferase I activity.
There is also the possibility to use transgenic plants of the invention displaying reduced or lacking N-acetyl glucosaminyl transferase I activity, which additionally have been transformed by the gene encoding the desired glycoprotein. In order to achieve this, constructs may be employed, which contain the gene encoding the desired glycoprotein under the control of a strong constitutive or inducible promoter, which is optionally tissue specific as well, and lead to the integration of the gene into the plant genome. Alternatively, the transformation may also be conducted by viral infection by means of a virus containing the gene for the desired glycoprotein for extrachromosomal propagation and expression of the gene. The glycoprotein may then be expressed in the respective host plant and obtained therefrom.
Naturally, as an alternative, the procedure may be such, that initially a transformation using an expression construct or virus containing the DNA encoding the glycoprotein is performed, and subsequently, another transformation with one or more of the antisense or sense constructs of the invention or with one or more viruses, containing the corresponding DNA, is performed. It is also possible to perform a simultaneous transformation using both constructs or using one virus containing the antisense or sense construct as well as the gene encoding the desired glycoprotein (piggyback version).
Within the scope of the present invention, there is also considered a viral overinfection of the transgenic plants according to the invention, in which integration of an antisense/sense construct and/or the gene encoding the desired glycoprotein into the genome has already occured, by viruses containing the antisense/sense construct and/or the gene encoding the desired glycoprotein, for an additional extrachromosomal propagation and transcription or expression, respectively, of this DNA. As a result, the concentrations of antisense or sense DNA, respectively, or of the expressed glycoprotein may be increased in the transgenic plant cells.
It may prove to be practical for the production according to the invention of glycoproteins with defined glycosylation, to use tissue specific promoters in such cases, where it is intended to obtain the desired glycoproteins specifically only from certain parts of a plant such as tubers or roots. Today, for a large variety of plant tissues, tissue specific promoters are available, which drive expression of foreign genes specifically only in these tissues. By way of example, tuber specific promoters such as patatin class I (ref. 26) and proteinase inhibitor II promoters (ref. 27) may be mentioned. Under certain conditions, both promoters exhibit expression also in leaf tissue, i.e. they can be induced by high metabolite contents (for example sucrose) and in the case of the proteinase inhibitor II promoter also by mechanical lesion or by spraying with abscisic or jasmonic acid, respectively.
The use of tissue specific promoters may also be indicated in cases, where the DNA sequence or the transcription products or translation products thereof according to the invention, respectively, which are employed for the transformation, turn out to be detrimental to certain plant parts, e.g. due to a negative influence on the metabolism of the corresponding plant cells.
As a representative target glycoprotein, human glucocerebrosidase may be used for the therapy of the hereditary Gaucher""s disease (ref. 25). In order to obtain human glucocerebrosidase (GC) with uniform and defined sugar residues, e.g. plants of the present invention which are transformed by means of antisense DNA, may be transformed with the gene encoding human glucocerebrosidase. For this purpose, the human glucocerebrosidase cDNA sequence (ref. 38) is modified at the 3xe2x80x2 terminus by means of PCR using gene specific primers in a manner, that the recombinant enzyme carries a C-terminal sequence extension encoding an affinity marker (e.g. AWRHPQFGG, strep-tag; ref. 39) and, optionally, also a protease recognition site (e.g. IEGR) between the GnTI enzyme region and the affinity marker. The GC-cDNA sequence thus altered is expressed in GntI antisense plants of the present invention by using a strong and optionally tissue specific promoter (e.g. for potato under the control of the tuber specific B33 patatin promoter), so that the enzyme synthesized in these plants exclusively carries well defined N-glycans. The affinity marker is intended to facilitate the enrichment of the recombinant enzyme from the transgenic plants. In this case, the expressed protein molecules (GC-strep molecules) bind to a streptavidin derivatized matrix via the affinity marker sequence and can be released therefrom by means of biotin or biotin derivatives. The removal from the strepatavidin derivatized matrix may also be carried out by means of catalytic amounts of a protease, which exhibits a specificity for the protease recognition site located between the GnTI enzyme region and the affinity marker. In this case, only the GnTI enzyme region is released from the matrix. This could be advantageous especially in that case, if the affinity marker sequence has a detrimental effect on the GnTI activity.
Due to their terminal mannose residues, the Man5GlcNAc2-glycans of the glucocerebrosidase obtained from the plants of the present invention will be recognized by macrophages as an uptake signal, and can thus directly be employed for the therapy of hereditary Gaucher""s disease. Currently, a therapy is only possible upon expensive isolation and deglycosylation of native glucocerebrosidase (ref. 25).
Accordingly, the production of recombinant glycoproteins may be highly facilitated by the use of plant GntI sequences compared to conventional methods, e.g. the chemical deglycosylation of purified glycoproteins, which is technically demanding (ref. 25), or a difficult and expensive production in GnTI deficient animal cell lines (ref. 7,10).